1. Field of the Invention
The present invention relates to an electroacoustic transducer.
2. Description of the Related Art
A device for generating ultrasonic waves for a vehicle is known from published German patent application document DE 10 2009 040 264 A1, in which an elongated ultrasonic transducer for generating thickness mode oscillations is coupled to a planar component of a vehicle, in particular a bumper. The ultrasonic transducer includes a sequence of piezo disks and contacting and insulating layers, the length of the piezo disks being matched to the desired ultrasonic wave frequency in air. The piezo disks are made of a piezoceramic fiber composite which includes fibers extending in parallel to the center axis of the particular piezo disk or in parallel to the longitudinal axis. The assembly of piezo disks is furthermore situated in a support device. This assembly, in turn, is attached on the bumper.
Published German patent application document DE 10 2005 037 724 A1 describes an ultrasonic transducer having a piezoelectric transducer body situated between two electrodes. The transducer body includes at least two volume areas made of piezoelectric semiconductor material, which are separated from each other by a boundary layer having the height h′. A lattice-shaped configuration having rod-shaped transducer segments is shown, in which one of the areas has a rod-shaped design. The described embodiment is to be used to implement an ultrasonic transducer having a short decay time and a pivotable beam.
An ultrasonic sensor is derived from published German patent application document DE 10 2008 018 110 A1, which includes a piezoceramic element which excites a bumper to oscillate and is provided within a piezoceramic module. The module is integratable into the bumper so as not to be visible from the outside. In addition, ribbing is formed around the piezoceramic module, so that the stiffness of the bumper is variable in this area and the mechanical impedance jump between the bumper areas is settable.
Published international patent application document WO 2011/090484 A1 describes an ultrasonic transducer in the form of a thickness oscillator. Proceeding from a disk-shaped piezoceramic including electrodes, at least one front element is provided, which has such a shape that a desired radiation pattern is created. The length of the transmission path corresponds to λ/4 at a resonance frequency fR, so that an amplification of the oscillation, proceeding from the piezoceramic disk, on the radiating surface is achieved. It is furthermore described that a λ/2 oscillator is preferred for some applications, which is more robust in particular with respect to temperature stability, susceptibility to soiling, and deposits. Such a λ/2 oscillator 1 is shown in FIG. 8. It includes a front element 40 and a rear element 60, the respective length of the front and rear elements, plus half of the thickness of the piezoceramic disk 50, corresponding to one quarter of the wavelength λ of the resonance oscillation. The advantage of such transducers is that they may be used as sensors for detecting surroundings of a vehicle or of a robot and may be integrated invisibly from the outside into body parts of the vehicle, for example the bumper, or into a lining part of the robot.
A λ/2 thickness oscillator 1 known from the related art, as shown in FIG. 8, has the disadvantage that it is very long. If the length is cut in half to send or receive with the aid of a λ/4 thickness oscillator, the surface of the piezoceramic disk situated opposite the front element must be connected to a sensor housing or a holding structure. If the piezoceramic is adhesively bonded to a substrate in a planar manner, or attached in another planar manner, for this purpose, this impedes the transverse contraction of the piezoceramic disk, whereby the sensitivity of the transducer is decreased.